Scientists knew our Sun was capable of producing incredible temperatures, but new research suggests they may have underestimated its thermal power.
According to researchers at the University of St Andrews, some solar flare particles are 6.5 times hotter than previously believed, offering a potential answer to a physics mystery that has persisted for half a century.
In sudden bursts, solar flares erupt from the Sun’s surface, producing atmospheric temperatures exceeding 10 million degrees. Such severe energetic events are a major driver of space weather, pushing X-rays and radiation in the direction of Earth, which poses a significant hazard to our near-Earth and ground-based electrical and communications infrastructure.
Examining Solar Flare Data
The University of St Andrews team, led by Dr. Alexander Russell, Senior Lecturer in Solar Theory from the School of Mathematics and Statistics, investigated how flares heat the ion and electron soup known as solar plasma to over 10 million degrees. Their findings suggest that the ion half of the plasma can reach temperatures beyond 60 million degrees.
As Dr. Russell’s team reviewed data from earlier studies of solar flares and plasma, they noticed that ions appear to heat to much higher temperatures than electrons.
“We were excited by recent discoveries that a process called magnetic reconnection heats ions 6.5 times as much as electrons. This appears to be a universal law, and it has been confirmed in near-Earth space, the solar wind, and computer simulations,” Dr Russell said. “However, nobody had previously connected work in those fields to solar flares.”
Rethinking Solar Physics
Traditional solar physics has long assumed that ions and electrons remain at the same temperature within solar plasma. However, significant advances—such as Japan’s Hinode satellite and NASA’s IRIS mission—have provided high-quality observations that are forcing researchers to reconsider long-held assumptions about our star.
“[IRIS] has provided unique spatially resolved observations of a hot (11 mega kelvin) flare line. Our experience with those observations directly motivated us to consider whether the solar observations might be best explained by a hotter ion temperature,” Dr. Russell told The Debrief.
“Redoing calculations with modern data, we found that ion and electron temperature differences can last for as long as tens of minutes in important parts of solar flares, opening the way to consider super-hot ions for the first time,” he added.
“What’s more,” Dr. Russell continued, “the new ion temperature fits well with the width of flare spectral lines, potentially solving an astrophysics mystery that has stood for nearly half a century.”
A Half-Century Solar Mystery
Since the 1970s, scientists have puzzled over why spectral lines produced by solar flares—the separated colors of the events observed in ultraviolet and X-ray wavelengths—are broader than expected. The prevailing explanation has been that turbulence within the flares distorted the wavelengths. However, after fifty years of searching for the source of this turbulence, many researchers have grown skeptical of the idea.
The new study presents a paradigm shift: ion temperature differences may be the real driver behind the puzzling line widths.
Looking Ahead
As researchers continue analyzing solar data, more questions may find answers—and likely more surprises will emerge.
“We’re already busy on the next steps: developing computer models of how flaring loops evolve when the ions are heated more strongly than the electrons; and gathering further observational evidence for hot ion temperatures using IRIS,” Dr. Russell explained.
The team is also looking forward to two future missions: NASA’s MUSE, a follow-up to IRIS scheduled for launch in 2027, and EUVST, a collaborative mission between Japan, the U.S., the U.K., and the EU that will provide seamless temperature coverage by measuring solar flare spectral lines.
“The two missions complement each other extremely well and together will provide an unprecedented understanding of solar flares,” Dr. Russell concluded.
The paper, “Solar Flare Ion Temperatures,” appeared in The Astrophysical Journal Letters on September 3, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.